1. Field of the Invention
This invention relates to a control valve for controlling injection passage of a direct on column injector, of the type having a valve body capable of rotating in a seat, to respectively open and close on injector passage for the injection needle insertion by means of a diametral through-channel, provided inside the valve body.
2. Description of the Prior Art
Valves of the mentioned type are applied to direct on column injectors as it is well known in the art. However in some cases, the use of direct injectors equipped with valves of the mentioned type, which are open during injection and closed at the end thereof leads to drawbacks, specially due to sample discrimination and loss of a sample portion.
In particular, when a so-called slow injection is carried-out, i.e. when the syringe piston is slowly lowered after the needle introduction into the injector, with open valve, the formation of a sample liquid cap may occur in correspondence with the free end of the injection needle, specially when the capillary column is at low temperature in correspondence to the injection zone. This liquid cap formation depends on different factors, among which solvent nature, column diameter and surface treatment of the glass forming the column itself, temperature of the injection zone, pressure and flow rate of carrier gas and amount of sample injected, of course together with injection speed. The introduction of carrier, which is performed upstream the zone of cap formation, causes a breaking of same, but a portion of the sample still remains on the needle tip. This sample portion is then drawn with the syringe outside the column, or may possibly remain inside the injector body, but in any case said portion, which is usually richer in heavy compounds, causes a discrimination of the sample, as the volatile components thereof are vaporized more quickly than the heavy ones during the needle insertion.
To overcome this drawback, it has been proposed to perform a quick injection, during which the syringe piston is quickly lowered. In this case, specially if the upstream end section of the column is heated, a vaporization zone in expansion and at high pressure is immediately formed downstream the needle tip, said zone tending to backwardly push the injected sample, inside the needle and above all into the passage between the needle and the column internal surface as well as into the injector passage. This involves, on one side, a backward ejection of small drops and vapour and, on the other side, a discrimination of heavy compounds inside the needle. Actually, vapours of volatile compounds and small drops of sample are produced, which are discharged in the atmosphere through the inlet passage for the syringe needle, as well as small drops which remain attached to the walls of the injector and of the initial section of the column, together with condensed vapours, with loss of the sample portion entrapped inside the injector body.
Another problem arising in the case of a direct on column injection, specially using certain solvents, is the problem of improving the work conditions at the injection end, by maintaining a certain exhaust to the atmosphere, in order to perform washing of the column and injector upper sections, as suggested by Grob. This exhaust is useful in some cases and Grob suggests to perform the same by keeping at least partially open the injection valve. However, this solution depends on the skill and sensibility of the operator, who not always proves to be up to the task.